Debugging

Steps to debugging

• Determine that you have a bug
• Isolate the bug's location
• Find the culprit code

Identifying a bug

• In order to find a bug, lots and lots of testing
• Answer the question: what is the behaviour that you think is buggy (in words).
• Why do you think that the behaviour is buggy?
  • Differs from given expected output?
  • Not what you were expecting, e.g., too slow, too much memory consumption, crashes, runs out of resources, never finishes, ...
• Try to pinpoint: under what circumstances does the bug appear
  • Try different inputs or outputs
  • Goal: find the smallest input possible that reproduces the bug!
• Be specific, and keep getting more specific until you find the issue!

There are many different strategies possible (the one detailed above is just one of them)

• Goal: Where in the code could the bug be? e.g., everything was working fine yesterday but something went wrong today, so perhaps the code written today is the first suspect
• Be creative: better to think of too many places than too few
  • Like a detective game. Can be quite fun when you find the issue
• Example: Identify different functions that could be the culprit
  • Then run each function separately (need to construct inputs to the function)
  • Print out parameters and return values
  • Use the debugger! e.g., gdb. See next week's lab assignment.

Finding the bug

• Once you have found the function, need to find the bug
• What does each line of code do?
  • Use print statements or the debugger to verify your assumptions
  • Explain each line of code to your partner or to an imaginary person
• Draw pictures - keep track of values in data structures and variable values

Using the debugger

• Add a breakpoint: program will pause at that line of code

  0  #include 
  1  int foo(int a)
  2  {
  3     a = a + 2;
  4     return a;
  5  }
  6
  7  int main()
  8  {
  9    cout << "hello\n";
  10   cout << foo(10) << endl;
  11   return 0;
  12 }
  

  We can ask the debugger to add a "breakpoint" at line number 3, for example. Show what will happen.